Hall thrusters generate thrust through the formation of an azimuthal electron current that interacts with an applied, quasi-radial magnetic field to produce an electromagnetic force on the plasma. These thrusters provide an attractive combination of thrust and specific impulse for a variety of near-earth missions and, in many cases, they allow for significant reductions in propellant mass and overall system cost compared to conventional chemical propulsion. The range of thrust and specific impulse attainable by Hall thrusters makes them applicable also to a variety of NASA science missions. Many such missions however require wider throttling and larger propellant throughput than near-earth applications. A critical risk associated with the application of Hall thrusters to science missions is their throughput capability. There are two major wear processes known to exist in Hall thrusters that can limit their applicability to such missions: erosion of the acceleration channel and erosion of the hollow cathode.
Multiple approaches have been pursued to reduce or eliminate this risk. For example, the high voltage, Hall accelerator (HiVHAc) incorporates an innovative discharge channel replacement technology as a means of extending its life. In 2008 the NASA-103M.XL version of HiVHAc accumulated more than 4,700 h at 700 V upon the completion of a wear test. More recently, a Qualification Life Test (QLT) of a different Hall thruster, the BPT-4000, was extended beyond 10,400 h. The BPT-4000 is nominally a 4.5-kW class thruster, and has fixed insulators and a magnetic design for high efficiency and long life. Post-test assessment of the wear data showed no measurable erosion of the acceleration channel walls from 5,600 h to 10,400 h indicating that the thruster reached an approximately “steady state” erosion configuration. The BPT-4000 QLT results were explained in a paper Mikellides, I. G., Katz, I., Hofer, R. R., and Goebel, D. M., de Grys, K., and Mathers, A., “Magnetic Shielding of the Channel Walls in a Hall Plasma Accelerator,” Physics of Plasmas, Vol. 18, No. 3, 2011, p. 033501, which suggests that if properly designed, the life of Hall thrusters can be extended to (or exceed) that of ion thrusters thereby retiring the risk associated with their throughput capability.
The BPT-4000 QLT has exceeded significantly the requirements for most commercial or military missions. However, because many NASA science missions require longer operational times, higher throughput, and a wider range of operating conditions, a rigorous understanding of the erosion physics was needed.
From the observed erosion trends in the BPT-4000, it was recognized that to fully understand such physics one must account, at minimum, for the 2-D distribution of the electric field near the eroding surfaces, the sheath physics, and the local topology of the magnetic field. To account for all these physics, it is required usually that the solution to an extensive system of governing laws for the Hall thruster plasma is obtained, in two or three dimensions. The importance of understanding the erosion physics in such topologies motivated the development of a Hall thruster plasma solver named “Hall2De.”
Hall2De is a 2-D computational solver of the laws that govern the evolution of the partially-ionized gas in Hall thrusters. The code is a descendant of OrCa2D, a 2-D computational model of electric propulsion hollow cathodes that employs a mix of implicit and explicit algorithms to solve numerically the plasma conservation laws in these devices. In Hall2De, excessive numerical diffusion due to the large disparity of the transport coefficients parallel and perpendicular to the magnetic field is evaded by discretizing the equations on a computational mesh that is aligned with the applied magnetic field. This magnetic field-aligned-mesh (MFAM) capability was largely motivated by the need to assess the life of Hall thrusters in complicated magnetic field topologies. A detailed description of the code has been provided by Mikellides, et al. (I. G. Mikellides, I. Katz, R. R. Hofer, and D. M. Goebel, Proceedings of the 31st International Electric Propulsion Conference, Ann Arbor, Mich. (Electric Rocket Propulsion Society, Fairview Park, Ohio, 2009), IEPC Paper No. 09-114).
There is a need for improved electric propulsion devices with greater throughput capabilities for many space applications.